Electric heater



Feb. 25, 1941. c. P. RANDOLPH ELECTRIC HEATER Filed Jan. 10, 1939 Fig.5. MOUNTING ALLOY FOR SPECIMEN m m IX SHEATH MATERIAL Inventor: CharlesR Randol h, Deceased B3 EQRandolph and Dorothy Saner, Executors,

HOURS ON HEAT amttowuty uuzfifimut 225 503 Thei r Attorneg Patented Feb.25, 1941 UNITED STATES PATENT orrlcr.

' ELECTRIC HEATER Application January 10, 1939, Serial No. 250,196

Claims.

This invention relates to electric heaters, and more particularly toelectric heaters of the sheathed type and has for its object animprovement in heaters of this character.

r; The invention has particular application to electric heaters of thesheathed type such as described and claimed in United States patent toC. C. Abbott No. 1,367,341, dated February 1, i921. Briefly, heaters ofthis type comprise a heating to element enclosed in a. metallic sheathand em= bedded in and supported in spaced relation with respect to thesheath by a densely packed layer of electrically insulating, heatconducting, refractory material, such as magnesium oxide. lit is to beunderstood, however, that this invention is not limited to this specifictype of heater but has general application in electric heaters having aresistance conductor encased in a metallic sheath.

It has been found in heaters oi the above-mentioned type that theinsulation resistance thereoi decreases during life. In some instancesthe im sulation resistance has decreased to 1% of its initial value in.about 1200 hours of heating.

In one of its aspects therefore this invention contemplates theprovision of an electric heater oi the sheathed type having a higher andmore permanent insulation resistance,

It has been round that if the inner surface of the metallic sheath isprovided with an integral or intimately bonded layer oi oxide the valueand his oi the insulation resistance are considerably improved. Theoxide layer may consist of an integ'ral layer of the oxides of themetals present in the sheath itself or it may be in the form of a layerof applied refractory oxides such as alumi-= num oxide, beryllium oxide,thorium oxide, mag nesium oxide, silica, zirconium oxide, chromiumoxide, or any other similar material either indi- 46 viciually or incombination. The essential reouirement with the last-mentioned materialsis that the material be intimately bonded to the in= ternal surface ofthe metallic sheath and be iormed in a relatively thick and smooth coat.

45 It is therefore an object of this invention to provide an. oridelayer on the inner surface of the sheath of an electric heater so as toincrease and maintain the insulation resistance of the heater elementduring its life.

50 It is a more specific object of this invention to provide an electricheater of the sheathed type in which the inner surface of the sheath isprovided with a developed integral oxide layer or intimately bondedapplied layer of oxide.

5 A further more specific object of this invention is to provide methodsby which the inner surface or the sheath of a sheathed type resistanceelement may be oxidized quickly and economically.

In the preferred embodiment 01' this invention a developed mteallicoxide layer consisting of an 5 integral layer of oxides of the metalswhich are alloyed in the sheath is provided on the inner surface of thesheath. The sheath may consist of any type oi alloy suitable for heatingelements operating at temperatures above l200 F., but a m metallicsheath having a composition within the range described and claimed inUnited States patcut to J. C. Sharp No. 2,034,539, dated March 1'],1936, is preferred; The developed integral metallic oxide layer isformed on such sheath material 15 by preliminarily preparing the innersurface of the sheath, for example by sandblasting, and then heating itfor a limited time at some predetermined high temperature whilesubjecting it to an oxygen atmosphere. In order to prevent oxlda 20 tionand scaling of the outside surf-ace oi the tube, the tube may be placedin a reducing or neutral atmosphere during the heating operation. It hasbeen found that the depth of oxide coating depends upon the temperatureto which the sheath 25 is subjected, the pressure of the oxygen, and thelength oi time of the treatment. An, integral layer oi oxide of anythickness substantially uniiormly formed on the inner surface of themetallic sheath will produce beneficial results but it is preferablethat a coating of oxide corresponding to a minimum gain in weight of thesheath of one tenth per cent (0.1%) shall be formed. It has been foundthat such a coating is sufiicient to maintain .the insulation resistancewell above the value of that of unoxiclized sheaths.

For a more complete understanding of this invention, reference should behad to thegaccompanying drawing in which Fig. l is a fragmentary view inelevation of an electric heater embodying this invention, portions ofthe heater being shown in section to illustrate structural details; Fig.2 is a cross-section of the heater element shown in Fig. 1; Fig. 3 is areproduction of a photomicrograph 01' a specimen of sheath materialshowing the structure of the developed or integral layer of oxide; andFig. 4 is a curve showing the improvement in insulation resistance ofoxidized units as compared with similar but unoxidized units.

Referring to Fig. 1, this invention has been shown in one form asapplied to an electric heater of the type described and claimed in theabovementioned Abbott Patent No. 1,367,341, as modifled and improved inthe United States patent to Oliver G. Vogel No. 2,094,480, datedSeptember 28,

, 1937. Briefly, this heater comprises a helical resistance conductorIII mounted within and centrally of a tubular metallic sheath I I.Conductor Ill preferably will be formed of a nickel chromium alloyhaving a nickel content of approximately 80% ands-chromium content ofabout 20% The resistance conductor I0 is embedded in and supported inspaced relation with respect to sheath l l by a densely compacted layerof a suitable heat conducting, heat refractory, and electricallyinsulating material I2, such as powdered magnesium oxide. The insulatingmaterial l2 may be compacted to a hard, dense mass in any suitablemanner, such as reducing the diameter of the sheath after the unit hasbeen assembled. This may be accomplished by either swaging or rolling.As described in the aforementioned Vogel patent, the characteristics ofthe heater element may be further improved by flattening the sheath onat least one side as shown in Fig. 2. v

The resistanceconductor III has its ends secured to terminals II, II,which project from the ends of the sheath and which have their innerends embedded in the insulating material I! so as to be supported inspaced relation with respect to the sheath l I. I

As previously explained, the sheath l2 may be composed of any suitablealloy capable of withstanding temperatures of 1200 F. or more for asustained period. It is preferred to use an alloy the constituents ofwhich fall within the range described and claimed in the aforementionedSharp Patent No. 2,034,539. One specific composition which has beenfound satisfactory is an alloy consisting of 79% nickel, 13% chromium,and approximately 8% iron.

An oxide layer 14 is provided on the inner surface of the metallicsheath H. In the preferred embodiment of this innvention the layer I4consists of an integral layer of the oxidesof the metals of which thesheath is composed which may be termed a developed metallic oxide layer.The method of oxidizing the sheath itself may be varied to aconsiderable extent depending on the thickness of oxide layer desiredand permissible production costs.

It has been found that a satisfactory coating of ortide could be formedon the sheath in the following manner. The tubular sheath is firstsandblasted on its inner surface to remove any protective scale formedthereon or foreign material such as dust, oil, etc. It is then oxidizedby passing oxygen at atmospheric pressure therethrough for from three tofive minutes while maintaining the tube at a temperature of 2200 F. Ifit is so desired, the outer surface of the tubular sheath may beprotected from oxidation during the heating by providing an inert orreducing atmosphere on the outside of the tube. The tube may be heatedin any satisfactory manner, but it has been found that the temperaturemay be controlled to a greater degree by passing current directlythrough the metallic sheath. This method has produced a coating of oxidewhich materially improves the insulation characteristics of the heaterelement. In addition, the method described does not produce anydetrimental scaling of the sheath element. While the oxidation might becarried on at a different temperature, either higher or lower, theaboveindicated oxidation temperature is preferred from the standpoint ofspeed of oxidation and ease of control.

of two atmospheres greatly facilitates the obtaining of a heavy oxidelayer in a relatively short time.

Tests have indicated that additional improved results might also beobtained by subjecting the sheath to a pickling or hydrogen sulphide gastreatment prior to oxidizing. Such preliminary treatment of the sheathis particularly desirable when heater elements of the type described inthe aforementioned Abbott patent are used in place of the improvedheater element described in the aforementioned Vogel patent. This is dueto the fact that the heater element of the Abbott patent has inherentlylower insulation resistance characteristics than the flattened type ofheater element described in the Vogel patent and hence the improvedresults due to such preliminary treatment are more pronounced whenheater elements of the Abbott type are employed. The pickling in onecase was carried out in a 50% solution of hydrochloric acid to whichapproximately 3% hydrofluoric acid was added. The solution wasmaintained at about 180 F. and the tubing treated for approximatelyone-half hour, after which it was bright-dipped in hot concentratednitric acid. With the hydrogen sulphide gas treatment, the tubing,after'sandblasting. was treated with hydrogen sulphide gas for fiveminutes at about 1100 F., followed by oxidation with oxygen for fiveminutes at 2200 F.

In Fig. 3 is shown a reproduction of a photomicrograph of a portion of atubular sheath pickled and oxidized as stated above. In this figure theunshacled portion represents the sheath material and the light mottledportion represents the alloy upon which the specimen was mounted. Thedark portion between the mounting alloy and the sheath material is thedeveloped or integral coating of oxide formed on the sheath.

With tubes oxidized in the manner indicated above, it has been foundthat such units after the first thousand hours of life test maintain aninsulation resistance at least twenty times higher than that ofuntreated units. The results obtained are shown graphically in Fig. 4.In obtaining these results the units tested were the same except thatthose giving the results shown in curve A were not oxidized, and thosegiving the results shown in surve B were oxidized. It is readilyapparent from these curves that oxidizing the inner surface of thesheath gives higher and more permanent values of insulation resistance.

It is not known exactly why this improvement results. Several theorieshave been advanced but none have been established as controlling. Onetheory is based upon micrographic studies. As shown in Fig. 3, itappears that an oxide film covers the surface of the metallic sheath andalso penetrates into the grain boundaries to a certain extent. It ispossible, therefore, that this oxide film or layer acts as a protectivelayer preventing diffusion of deleterious gases or vapors from the metalinto the insulation during life.

Another possible explanation is that the materials added to the sheathby oxidation can react with the magnesium oxide refractory material. Inthis way a bond between the metal and the insulation isformed, thusassuring better thermal I contact and restricting migration ofimpurities from the metal into the magnesium'oxide.

Neither of these theories is a complete explanation of the resultsobtained, but is substantiated by tests that the oxidation of the innersurface of the tubular sheath gives higher and more permanent values ofinsulation resistance throughout the life of the heater. As previouslystated, an intimately united layer of oxide of anythicknesssubstantially uniformly 4 formed on the inner surface will produce theaforementoned beneficial results but it is preferred that a coating ofoxide corresponding to a minimum gain in weight of the sheath of 0.1%

- be formed on the sheath. With a sheath composed of an alloy theconstituents of which fall within the range described and claimed in theaforementioned Sharp Patent No. 2,034,539 and particularly the preferredspecific composition referred to above a gain in weight of 0.l,%corresponds to a coating of oxide of approximately 4.6 milligrams persquare inch of area.

While in the preferred embodiment the oxide layer is formed from themetals of the sheath itself, it has also been found, as previouslystated, that beneficial results may be obtained by using an appliedrefractory oxide coating or layer of various other oxides, such asaluminum oxide, beryllium oxide, thorium oxide, magnesium oxide, silica,zirconium oxide, chromium oxide, or similar materials, eitherindividually or in compounds. These are applied to the inner surface ofthe tubular sheath in any suitable man- .ner so as to bond intimatelythe refractory oxide to the metal and thereafter provide a coating whichtightly adheres to the metal surface.

In the case of alumina, for example, the tubular sheath is firstsandblasted in order to clean the surface thereof and provide anextended surface area. It is then coated with fired, finely groundalumina which is suspended in a suitable carrier. The particularsolution employed was a mixture of 68% alumina suspended in an organicmaterial. After this solution is applied to the sheath it is air driedand then heated in an oven from room temperature to about 250 F., atwhich temperature it is held for about one hour. It is then heated toabout 400 F. and held at this temperature for another hour. The tubesare then heated to about 2200 F. and maintained at this temperature forapproximately twenty minutes.

The method for applying beryllium oxide is substantially the same asthat used in applying a refractive coating of alumina except that asolution comprising about 54% beryllium oxide suspended in an organiccarrier is employed. In each of these methods the application of therefractory coating may be facilitated by passing oxygen through thetubing during the final heating step of the method.

By employing either of the methods just described it was found that anevenly distributed refractory coating could be intimately bonded to theinner surface of the metallic sheath and that sheaths so coated whenused in electric heater elements would produce units having higherinsulation resistance.

While particular embodiments of this invention have been shown anddescribed, modifications thereof will be apparent to those skilled inthe art. It is desired, therefore that this invention should not belimited to the particular constructions and methods shown and described,and it is intended in the appended claims to cover all modificationswithin the spirit and scope of this invention.

What we claim as new and desire to secure by Letters Patent in theUnited States is:

1. In a sheathed type electric heater, a coiled resistance unit embeddedin a compacted mass of electrically insulating, heat conductingrefractory material, a metallic sheath encasing said element and saidrefractory material, and a refractory oxide coating intimately united tothe inner surface of said sheath and engaged by said refractorymaterial.

2. A sheathed type electric heater comprising a coiled resistance unitembedded in a compacted mass of electrically insulating, heat conductingrefractory material, a metallic sheath encasing said element and saidrefractory material, and a developed metallic oxide layer integrallyformed on the inner surface of said sheath and adapted to be engaged bysaid refractory material.

3. A sheathed type electric heater comprising a coiled resistance unitembedded in a compacted mass of electrically insulating, heat conductingrefractory material, a metallic sheath encasing said element and saidrefractory material, and an applied refractory oxide coating intimatelybonded to the inner surface of said sheath and adapted to engage saidrefractory material.

4. Asheathed type electric heater comprising a tubular sheath the innersurface of which is provided with a tightly adhering refractory oxidecoating, a resistance element within said sheath, and an electricallyinsulating heat conducting mass compacted between the oxide coating ofsaid sheath and said heating element.

5. In an electric heater of the sheathed type, a coiled resistanceelement, a partially flattened metallic sheath formed of alloyed metalsencasing said resistance element, a developed layer of the oxides ofsaid alloyed metals integrally formed with the inside surface thereof,and a mass of heat conducting electrically insulating material compactedbetween said sheath and said resistance element and embedding saidresistance element.

6. An electric heater of the sheathed type comprising a metallic sheath,an electric resistance element arranged centrally of said sheath, atightly compacted mass of insulating material embedding said resistanceelement, and a layer of applied refractory oxide arranged between saidinsulating material and said sheath and intimately bonded to the innersurface of said sheath.

7. An electric heater element of the sheath type comprising a metallicsheath, an integral covering of oxide on the inner surface of saidsheath, said oxide covering comprising oxides of the metals of which thesheath is composed, a resistance element within said sheath and embeddedin a tightly compacted mass of electrically insulating heat conductingrefractory material.

8. A metallic sheath for an electric heater adapted to encase aresistance element embedded in an electrically insulating heatconducting mass, said sheath being composed of an alloy comprisingapproximately 79% nickel, 13% chromium, and 8% iron, and havingdeveloped on its inner surface an oxide coating comprising oxides ofnickel, chromium, and iron.

9. In an electric heater of the sheathed type, a metallic sheath, anintegrally formed oxide coating on the inner surface of said sheath, a

resistance element within said sheath, and a mass of refractory materialcompacted between said resistance element and said oxide coating wherebyan electric heating unit having high insulation resistance is provided.

10. A sheathed type electric heater having high insulation resistancecomprising a tubular sheath the inner surface or which is provided withan intimately bonded layer of applied refractory oxide, a coiledresistance element within said sheath, and a compact mass ofelectrically insulating heat conducting refractory material within saidsheath embedding said resistance element.

E. C. RANDOLPH. DOROTHY SANER, Executor-s of the Estate of Charles P.Randolph,

Deceased.

